Capacity control for absorption refrigeration



M. K. ROHRS 3,225,556

CAPACITY CONTROL FOR ABSORPTION REFRIGERATION Dec. 28, 1965 2 Sheets-Sheet 1 Filed Feb. 24, 1964 COOLING rowan MARVIN K. ROHRS INVENTOR. BY fbwmawa Dec. 28, 1965 M. K. ROHRS 3,225,556

CAPACITY CONTROL FOR ABSORPTION REFRIGERATION Filed Feb. 24, 1964 2 Sheets-Sheet 2 COOLING TOWER MARVIN K. RGHRS IN VEN TOR.

United States Patent 3,225,556 CAPACITY CONTROL FOR ABSORPTIGN REFRIGERATION Marvin K. Rohrs, Fanwood, N.J., assignor to Worthington Corporation, Harrison, N.J., a corporation of Delaware Filed Feb. 24, 1964, Ser. No. 346,774 11 Claims. (Cl. 62-141) This invention relates to an absorption refrigeration system connected to circulate a vaporizable saline solution made up of a refrigerant and an absorbent material. It relates in particular to an arrangement of parts for controlling the capacity of the refrigeration system in response to the load imposed thereon.

Absorption systems of the type contemplated possess many desirable operating and economic advantages. They are for example relatively simple in construction, and be cause of the need for but few moving parts, only minor maintenance is required. Economically, an absorption system or machine is particularly desirable where, among the utilities available, combustible gas or steam are most accessible as a source of energy.

In a system of the type herein contemplated, saline solution is circulated in varying degrees of concentration of the solute. Usually a salt such as lithium bromide as the absorbent is dissolved in water as the solvent. Water as the refrigerant is then circulated through the system in both liquid and vapor phase.

An absorption system, as with any controllable refrigeration system, must be sensitive to changes in loading conditions and also be adapted to rapidly modify its operating characteristics to accommodate changes in load.

It is known in the prior art that part load operation is obtained by concentration control of solution being circulated in the system absorber which is effected basically by control of the rate at which refrigerant is driven from the liquid saline solution in the generator. In systems having steam heated generators there is often no control of the incoming flow of steam, the sole control being the conventional regulation of steam pressure such that the latter does not exceed a desired pressure. In the usual case of steam being supplied by a heating boiler, regulation is part of the operation of the boiler plant and not of the absorption refrigeration system. In absorption systems heated by means or" hot water delivered to the generator the flow of hot water may be controlled to give a set discharge temperature. In the instance of lithium bromide absorption refrigeration machines, this discharge hot water temperature is on the order of about 240 F.

T 0 facilitate the present description without unduly limiting the invention, it is assumed that the source of heat supplied to the generator is at constant temperature. Such an assumption is relatively close to actual practice in the case of steam heated systems. In practical systems, control at part load is effected, among other ways, by varying the flow of Weak solution circulating from the absorber to the generator and back to the absorber. The less the flow of weak solution to the generator, the less will be the quantity of refrigerant driven from the saline solution, although the relationship is not in practice proportional.

In order to attain good operating economy at part load it is necessary that the quantity of heat energy supplied to the system be adjusted substantially proportional to load. This alteration in the heating rate can be achieved by a reduction in the driving force represented by the temperature difference between the heat source, and the brine solution being heated. Since the heat source is at constant temperature, the reduction in temperature difference is effected by raising the temperature of the brine solution.

To achieve a simple efiicient means for capacity control, the present invention provides a novel arrangement and connection of elements in the refrigeration system to maintain a substantially constant flow of solution to the absorber while varying the flow rate of solution entering the generator. Simultaneously, the relative concentration of solution entering the absorber is controlled to maintain a sufiicient degree of concentration in accordance with the cooling load placed on the system.

It is therefore an object of the invention to provide means for readily varying the cooling capacity of an absorption refrigeration system in accordance with the load thereon.

It is another object of the invention to provide means for maintaining a substantially constant flow of solution to the system absorber while regulating the solution concentration,

It is still another object of the invention to provide means for diverting a stream of weak solution leaving the absorber as to bypass both the system generator and heat exchanger, thereby limiting the degree of solution reconcentration.

It is a still further object of the invention to provide means for controlling the flow rate of concentrated solution leaving the system generator in response to variations in the flow rate of weak solution leaving the absorber.

These and other objects of the invention will become apparent to one skilled in the art from the following description of the invention made in conjunction with the accompanying drawings.

FIG. 1 is a diagrammatic illustration of the invention showing the interconnection and arrangement of cooperating elements.

FIG. 2 is similar to the arrangement in FIG. 1, showing an alternate means for controlling the flow of solution passing from the absorber to the generator for conconcentration in the latter.

FIGURE 1 illustrates diagrammatically a system of the type contemplated which includes an absorber 10, an evaporator 11, a condenser 12, a generator 13 and a heat exchanger 14, all'connected in a closed circuit containing saline solution made up of an absorbent material and a refrigerant. Heat exchanger 14 is disposed in the system in a manner to pass saline solution in varying degrees of concentration, into heat exchange contact when flowing from absorber to generator, and from generator to absorber. Heat exchanger 14 is connected to the outlet side of generator 13 and under normal operating conditions, receives a hot stream of concentrated solution from the latter for introduction to an ejector 16 interposed in a pumping circuit. The latter circuit connects the lower part of the absorber with the upper portion for spraying intermediate strength solution into the absorber.

Conduit means connected to generator 13 carries a stream of strong solution into heat exchanger 14. Conduit means connected to absorber is also communicated with a pumping circuit to introduce and intermix hot concentrated solution with weak solution prior to the latter being recirculated to the absorber.

In the following description, the term flow spoilage, refers to the addition of weak solution normally drawn from the absorber, to a stream carrying concentrated solution leaving the generator. This spoilage occurs at a point in the system where concentrated solution has been discharged from the system heat exchanger 14 for contacting cooler solution leaving the absorber.

The term solution, or brine solution is hereinafter used to designate a liquid made up of absorbent and vaporizable refrigerant being passed through the system. This is in contrast to pure refrigerant which flows in both liquid and vapor phase. The saline solution however, although being in the liquid phase is nonetheless subject to changes in concentration to meet any change in load imposed on evaporator 11, which circulates water chilled to an outlet temperature between about 40 and 50 degrees F.

The terminology weak solution as herein referred to, defines a condition of the above solution in which there is a predominance of refrigerant so that functionally the solution is weak in absorbing properties. The term strong solution on the other hand defines a solution condition characterized by a lesser amount of refrigerant so that the solution possesses greater absorption properties.

While various combinations of absorbent and refrigerant may be circulated through the present system, the art has demonstrated that a mixture of water as a refrigerant and lithium bromide as a salt is highly desirable.

Referring to FIGURE 1, absorber 10 and evaporator 11 are disposed within a low pressure shell enclosure 17 whereby water vapor as the refrigerant, passes upwardly to absorber 10 into contact with sprayed streams of brine solution.

Generator 13 and condenser 12 are positioned in a higher pressure shell 18, and relatively disposed with respect to each other to pass vaporized refrigerant to the condenser 12 for contacting water circulating coils 19. In generator 13, refrigerant is boiled from weak brine solution to concentrate the latter. Released refrigerant vapor passes upwardly and is condensed in the indirectly cooled condenser 12. Refrigerant condensate is then pumped or forced from high pressure shell 18 through conduit 21 to evaporator 11 in which at least a portion of the condensate flashes.

Weak solution which is gravity fed from sump pan 22 in absorber 10 by way of line 23, and heat exchanger 14, to generator 13, normally varies in concentration between about 55 and 61 percent. The flow rate and volume of weak solution is adjusted however, by operation of control valve 48, as required to maintain the concentration of absorbent to a value necessary to meet the cooling load imposed on the system.

Under normal operating conditions weak solution enters generator 13 at a concentration of from about 55 to 61 percent and leaves in a heated condition at about 66 to 69 percent concentration. Absorber 10 includes nozzles 26 which direct sprays of solution against tube bundle 27 to form in sump 22 a pool of weak solution. It is understood that in determining the above concentrations, the solution may contain other ingredients such as rust inhibitors and wetting agents in sufliciently small amounts not to appreciably alter the concentrations.

Condenser 12 and evaporator 11 define elements of the system holding essentially pure refrigerant in both liquid and vapor phase to function in a manner hereinafter described.

Structurally, the apparatus embodying the present absorption system includes the elongated pressure tight shell 17 supported by legs or other means not shown. Shell 17 is preferably of unitary welded construction, horizontally disposed, and having end plates including removable access openings defining a substantially airtight enclosure. The shell 17 includes an upper and lower section formed by sump pan 24 which is welded or fastened by other means along the ends and one side to the shell 17 inner wall and shaped to form sump 22.

Solution and refrigerant pumps 29 and 31 respectively forming a part of the pumping circuit, are supported on a shell outer surface. If however, the pumps are of the hermetic type they may be disposed within one of the shells in direct contact with liquid being pumped.

Absorber 1t) and evaporator 11 are positioned in upper shell 17, separated by the sump pan 24 having an upturned edge 32. Edge 32 is faced inwardly of and spaced from the shell wall, thereby providing a spray guard and passage for vapor rising from the evaporator.

Cooling coil or tube bundle 27, positioned above pan 24, is provided with an inlet 33 and outlet 34 for circulating water or other cooling medium. Cooling liquid is thus circulated to coil 19 in condenser 12, thence through a cooling tower not shown, and subsequently recirculated back into the system by means of a pump or other liquid moving element.

Heat exchanger 14 as presently shown, may be of the shell and tube type, including a coil 36 disposed to contact weak and strong solution as the latter move through the system. The heat exchanger is physically disposed contiguous with the lower side of shell 18, being preferably in direct heat exchange contact with a wall of the normally heated generator 13.

The liquid circulating portion of the system, including absorber 10, generator 13, and heat exchanger 14, embodies a plurality of lines carrying solution and arranged in a manner to control operation of the system in accordance with the cooling load. Under normal operating conditions, varying amounts of solution are collected in sump 22 after passing across the surface of tube bundle 27. A plurality of lines communicate with sump 22 having the respective inlets thereof disposed to receive streams of solution under varying liquid heads. These lines as seen in FIG. 1, open into the sump 22 at different elevations with respect to solution held in the sump, thereby providing a weir effect to solution streams leaving the absorber.

The shell side of heat exchanger 14 is connected at opening 37 to generator 13, receiving concentrated solution from the latter. The outlet side of heat exchanger 14 shell, is communicated through means at the lower side thereof forming a passage 38, to a receiver 39.

Receiver 39 includes a vapor tight enclosure having a plurality of inlets carrying solution thereto, and an outlet for discharging solution. A partition 41 in receiver 39 defines adjacently positioned chambers 42 and 43. A discharge opening 44 at the lower side of chamber 43 passes solution in varying degrees of concentration in a manner to be hereinafter explained.

Chamber 42 as seen in FIGURE 1, holds an amount of concentrated solution to the upper level of partition 41 thereby providing that the shell side of heat exchanger 14 will remain substantially full of concentrated solution under all operating conditions. This feature further permits solution to be held in the generator and heat exchanger when the system is shut down thus facilitating removal or repair of such elements as pumps 29 and 31 and ejector 16.

The circuit communicating absorber 10 with generator 13 and heat exchanger 14, functions to pass weak and strong solution therebetween. A portion of the weak solution leaving absorber 10 is directed through line 23 to heat exchanger 14 and to generator 13 for reconcentration. A second portion of the solution is bypassed through line 47, valve 48 and line 49 around both generator 13 and the heat exchanger 14, and thence recirculated to absorber 10.

Referring to FIGURE 1, line 23 in the pump circuit is positioned with an inlet at sump 22 to receive weak solution. Line 23 connects to the inlet side of coil 36 in heat exchanger 14, such that weak solution is brought into heat exchange contact with hot concentrated solution held in the heat exchanger 14 shell side. The second line 47 connected to absorber sump 22, and to a valve 48, communicates through line 49 to inlet 51 of receiver 39.

A third line 52 having an inlet connected to absorber sump 22, receives cool, weak solution from the latter for recirculation to absorber spray nozzles 26. Line 52 is connected to suction 53 of pump 29. Line 54 connected to the discharge of pump 29, receives solution which is then carried to spray nozzles 26 in the absorber for distribution therein. Line 56 also connected to the discharge of pump 29 connects through valve 57 and line 58 to the inlet of ejector 16.

Ejector outlet 59 is connected through line 61 to line 52 for introducing a stream of intermediate concentration for mixing with the stream of weak solution leaving absorber 10 through line 52.

Conduit means 62 communicates the center port of ejector 16 with outlet 44 of receiver 39, permitting solution held in chamber 43 to mix with solution passing through ejector 16.

The present invention as described, is directed particularly to means for regulating the capacity of the absorption system by controlling the flow rate, and the flow direction, of weak solution leaving absorber 10. The system however may include other control elements known in the art for varying the operating characteristics and functions of other elements in the system in response to variable conditions.

For example, coils 27 and 19 in absorber 11 and condenser 12 respectively, are interconnected through a cooling tower or similar means not presently shown, to define a cooling water circuit. Valve 65 connected into the cooling water circuit, is adjustable through sensing means 66 to vary the rate of cooling water fiow in response to the condition of refrigerant condensate leaving condenser 12, or in accordance with ambient conditions which would effect condenser cooling water temperature.

Similarly, heat supplied to generator tube bundle 67 may be in the form of steam or other fluid. Heating media is thus introduced through line 63 to boil solution held in generator 13, causing refrigerant to pass off in the form of vapor through passage 69 to condenser 12. Heating medium after passing through the generator 13 heating tubes 67, is directed through line 71 to a supplementary heating coil 72 at the lower side of heat exchanger 14. Thereafter steam condensate or heating medium, is carried through line 73 and control valve 74,

thence returned to the heat source.

FIGURE 2 illustrates diagrammatically an alternate embodiment of the system described in FIGURE 1 thus, the majority of the numerical designations as applied to FIGURE 1 will apply equally as well to FIGURE 2 with the following exceptions. In FIGURE 2, line 4-8 is connected to sump 22 in the absorber 11 for directing a stream of weak solution from absorber 10, to a threeway valve 76. Valve 76 includes a plurality of discharge outlets, one of which is connected through line 77 to inlet 51 of receiver 39. A second outlet is connected through line 78 to the inlet of heat exchanger coil 36.

Valve 76 may be manually operated in achieving the present purpose, but is preferably automatically operable in response to a condition in the system such as the condition of chilled liquid leaving the evaporator as determined through sensing means 79 associated with line 81 leaving evaporator 11. Thus, valve 76 passing weak solution from absorber 19, is adjustable to simultaneously vary a stream of Weak solution fed either to the heat exchanger 14- or to receiver 39.

Operation of the system Referring again to FIGURE 1, line 23 connected to sump 22 carries weak solution from absorber 10 to be introduced to generator 13 for heating and reconcentration. The pressure within generator 13, although below atmospheric, is normally substantially higher than the pressure in absorber 11, thus, introduction of liquid into generator 13 must be under a predetermined minimum head of liquid carried in line 23 sufficient to overcome pressure in the generator.

Line 52, also connected to the lower part of sump 22 carries solution not opposed by pressure in any other part of the system, and will consequently pass a substantially constant stream from absorber sump 22 into the suction of pump 29. Thus, return of intermediate strength solution through line 54 to the absorber nozzles 26, will also be at a substantially constant rate flow to maintain the tubes of absorber tube bundle 27 in a wetted condition under all circumstances of loading.

Adjustment of the capacity of the present system to meet a particular cooling requirement, is achieved by varying the concentration of solution passed through line 54 for introduction to absorber 10. As the cooling load on the system increases, a more concentrated solution is fed to absorber 10. Conversely, as the load conditions lessen, although the flow rate of solution passed to the absorber 19 is substantially constant, solution concentration will be reduced.

Variation in solution concentration is achieved by regulating the concentration of solution contained in chamber 43 prior to said solution entering ejector 16. Chamber 43 is connected as shown in FIGURE 1 to the inlet of ejector 16. Thus, with a pressurized stream of solution passing through the ejector as a motivating force, under all loading conditions solution from chamber 43 will be induced into the ejector, mixed with intermediate strength solution, and thence recirculated through pump 29 and line 54 for spraying into absorber 10.

Since the overall flow of solution to the absorber 10 will be substantially constant, and the flow of solution through conduit 62 into ejector 16 will be substantially constant, the concentration of the solution stream passing through conduit 62 will be the primary factor in determining the strength of solution that reaches absorber 10.

Under full load conditions, it is desirable that solution leaving chamber 43 be at maximum concentration. Thus, valve 48 is adjusted in response to the condition of chilled water at evaporator outlet 81 to reduce or stop weak solution flow into 49 thereby eliminating, or reducing dilution of strong solution within chamber 43.

At intermediate load conditions, valve 48 is partially opened to introduce weak solution through line 49 from sump 22, to mix with strong solution held in chamber 43. Such mixing forms a solution of intermediate concentration.

For normal, continuous operation then, valve 48 serves to reduce the circulation of solution between absorber and generator as load on the system is reduced. This regulation is inherent in the arrangement of conduits and weirs connecting the absorber and generator. As shown in FIG. 1 as valve 48 opens to increase flow through conduit 49 since the fiow in passage 62 is constant as a characteristic of eductor 16, the flow through passage 38 is reduced. Since the only source of replenishment of solution to absorber 1i resides in strong solution leaving 38, the rate of withdrawal of weak solution from absorber 10 to be concentrated in generator 13 is established by the rate of replenishment and thus the rate of recycling solution between absorber and generator is controlled by actuation of valve 48.

As valve 48 opens flow through line 49 increases, and by an equal amount the flow through passage 38 decreases thus causing an equivalent decreasse in flow withdrawal from the absorber for regeneration in generator 13. The control of this recycling is important in order to give good steam economy. At light loads only a small amount of recycling is required whereas at high loads, the maximum amount of recycling is necessary to achieve the required regeneration. Thus, the function of lines 47, 49 and valve 48 in the sequence of operation, is to regulate the system cooling capacity. The physical regulation of solution is referred to as the spoiling of the ejector 16 operation since the latter no longer induces strong solution from chamber 43 but rather induces mixed, intermediate strength solution for recirculation to absorber 10.

At very light or no load conditions, valve 48 is fully opened thereby passing an increased or maximum flow of weak solution into chamber 43. Solution then directed by way of outlet 44 through conduit 62 for mixing with the weak solution circulatory system, will be essentially a relatively weak concentration.

One effect of this spoilage of strong solution flow from generator 14, is to raise the vapor pressure within generator 10 thereby reducing the rate at which refrigerant is boiled from weak solution. Consequently, the overall operation of the absorption system will be adjusted to conform to the low load conditions imposed thereon.

In the embodiment of the system shown in FIG. 2, operation is essentially similar to operation of the system described with respect to FIG. 1. However, actuation of the three way valve 76, achieves a dual function. Said valve controls flow of weak solution to chamber 43 from the absorber 10 for the purpose of spoiling performance of ejector 16 and thereby regulating system capacity. A secondary function achieved by adjustment of valve 76 is to regulate weak solution flow to heat exchanger 14 approximately inversely in proportion to the rate of solution flow to chamber 43.

Thus, although the total volume of solution leaving absorber 10 will be substantially constant under all loading conditions, the degree of ejector spoilage, and the amount of solution to be reconcentrated in the generator, are regulated by proportioning of the flow stream. Valve 76 as shown, is adjustable in response to a condition in the circuit representative of a change in load conditions. As presently shown, sensing means connected to valve 76 may be responsive to the temperature of chilled fluid leaving the evaporator 11.

Although the single valve 76 is here shown as being controlled to vary flow of solution from absorber 10, it is also within the scope of the invention that similar control may be effected by a plurality of valves so spaced and separately controlled to regulate solution flow to chamber 43 and to the heat exchanger respectively.

From the foregoing, it is clear that the present invention provides a simple yet effective control for varying the refrigeration capacity in accordance with load on the system.

It is understood however that certain modifications and changes might be made in the system without departing from the spirit and scope of the invention.

What is claimed is:

1. In an absorption system including an absorber holding weak solution, a generator holding hot strong solution, an evaporator connected to chill a fluid passing therethrough, and a condenser, said respective members being connected to form a closed system circulating a saline solution consisting of vaporizable refrigerant and an absorbent material, the combination therewith of:

(a) a pumping circuit including a pump connected to the absorber and circulating weak solution received from said absorber for introduction to the latter,

(b) a heat exchanger having inlet and outlet means,

(c) at least one of said heat exchanger inlet means being connected to the absorber and receiving weak solution therefrom for introduction to the generator,

(d) a first line communicating said generator with another of said heat exchanger inlet means and carrying hot solution into heat exchange contact with weak solution therein,

(e) a receiver connected to the downstream side of the heat exchanger receiving and holding cooled concent-rated solution,

(f) a second line communicating said absorber with the receiver and carrying a stream of weak solution for introduction to the latter to intermix with solution contained therein, and

(g) conduit means communicating said receiver with said pumping means for introducing a mixture of intermediate strength solution from the receiver with weak solution in said pumping circuit.

2. The combination defined in claim 1 including:

(a) a panel disposed in said receiver forming communicated first and second compartments,

(b) said first compartment being connected to the heat exchanger outlet and receiving cooled solution therefrom,

(c) said second compartment having an outlet connected to said conduit means for passing solution to the latter.

3. The combination defined in claim 2 wherein said panel in the receiver is positioned therein to maintain a supply of cooled solution in the heat exchanger under all loading conditions on the system.

4. In the combination defined in claim 2 wherein said panel is fixed in a substantially upward position thereby forming an overflow passage at the upper side thereof for overflowing solution from the first compartment into the second compartment.

5. In the combination defined in claim 1 wherein said second line communicating said absorber with the receiver includes control means therein for regulating the flow rate of solution introduced to the latter.

6. In the combination defined in claim 5 wherein said control means include a flow control valve operable to regulate solution passing to the receiver.

7. In the combination defined in claim 2 wherein said second line is connected to the receiver second compartment for introducing a controlled stream of weak solution for mixing with solution entering said second compartment from the first compartment.

8. In the combination defined in claim 7 wherein said flow control valve is operable in response to the temperature of chilled liquid at the evaporator to regulate the flow rate of solution entering the receiver compartment.

9. In the combination defined in claim 6 wherein said pumping circuit includes:

(a) an ejector having an inlet connected to the pump means discharge and receiving a stream of motivating fluid from the latter,

(b) said ejector having an inlet connected to said second compartment and receiving a stream of solution therefrom for mixing with the stream of motivating liquid.

10. In the combination defined in claim 6 wherein said valve includes:

(a) means forming an inlet connected to the absorber and receiving weak solution therefrom,

(b) means forming an outlet connected to said receiver and introducing a stream of weak solution thereto,

(c) means forming another outlet connected to at least one of said inlet means in the heat exchanger.

ll. In an absorption refrigeration system including an absorber holding weak saline solution, a generator holding hot saline solution, an evaporator chilling a fluid passing therethrough, and a condenser, said elements being connected to circulate said saline solution consisting of an absorbent and a refrigerant:

(a) a pumping circuit including:

(1) a pump in said system circulating intermediate strength solution to the absorber and having a suction inlet receiving weak solution therefrom and a discharge for discharging the weak solution,

(2) an ejector having a first inlet connected to the pump discharge and passing a stream of motivating weak solution and a second inlet adapted to receive induced intermediate solution therein to mix with the motivating weak solution,

(b) a heat exchanger having inlet and outlet means connected in said system and passing streams of Weak and strong solution in heat exchange contact,

(c) said heat exchanger having an inlet means connected to the generator and receiving hot concentrated solution therefrom,

(d) conduit means communicating said heat exchanger outlet means with the second inlet of the ejector of the pumping circuit and carrying a cooled concentrated solution stream to the latter for mixing in the ejector,

(e) flow control means communicating said absorber 10 with said conduit means and regulating a variable flow stream of Weak solution from said absorber for introduction thereto to the solution stream carried in said conduit means, prior to mixing of the latter mentioned solution stream in the ejector of the pumping circuit.

References Cited by the Examiner UNITED STATES PATENTS ROBERT A. OLEARY, Primary Examiner. 

1. IN AN ABSORPTION SYSTEM INCLUDING AN ABSORBER HOLDING WEAK SOLUTION, A GENERATOR HOLDING HOT STRONG SOLUTION, AN EVAPORATOR CONNECTED TO CHILL A FLUID PASSING THERETHROUGH, AND A CONDENSER, SAID RESPECTIVE MEMBERS BEING CONNECTED TO FORM A CLOSED SYSTEM CIRCULATING A SALINE SOLUTION CONSISTING OF VAPORIZABLE REFRIGERANT AND AN ABSORBENT MATERIAL, THE COMBINATION THEREWITH OF: (A) A PUMPING CIRCUIT INCLUDING A PUMP CONNECTED TO THE ABSORBER AND CIRCULATING WEAK SOLUTION RECEIVED FROM SAID ABSORBER FOR INTRODUCTION TO THE LATTER, (B) A HEAT EXCHANGER HAVING INLET AND OUTLET MEANS, (C) AT LEAST ONE OF SAID HEAT EXCHANGER INLET MEANS BEING CONNECTED TO THE ABSORBER AND RECEIVING WEAK SOLUTION THEREFROM FOR INTRODUCTION TO THE GENERATOR, (D) A FIRST LINE COMMUNICATING SAID GENERATOR WITH ANOTHER OF SAID HEAT EXCHANGER INLET MEANS AND CARRYING HOT SOLUTION INTO HEAT EXCHANGE CONTACT WITH WEAK SOLUTION THEREIN, (E) A RECEIVER CONNECTED TO THE DOWNSTREAM SIDE OF THE HEAT EXCHANGER RECEICING AND HOLDING COOLED CONCENTRATED SOLUTION, (F) A SECOND LINE COMMUNICATING SAID ABSORBER WITH THE RECEIVER AND CARRYING A STEAM OF WEAK SOLUTION FOR INTRODUCTION TO THE LATTER TO INTERMIX WITH SOLUTION CONTAINED THEREIN, AND (G) CONDUIT MEANS COMMUNICATING SAID RECEIVER WITH SAID PUMPING MEANS FOR INTRODUCING A MIXTURE OF INTERMEDIATE STRENGTH SOLUTION FROM THE RECEIVER WITH WEAK SOLUTION IN SAID PUMPING CIRCUIT. 